Long-term availability of liquid hydrocarbon fuels for transportation and military propulsion operations remains a present concern in view of increasing demand and shrinking energy resources. It is known that some nano-particle energetic materials can be employed as fuel additives to enhance combustion properties of solid fuels. Nano-particle energetic materials can advantageously increase a solid fuel's energy density and/or can enable higher engine performance through increased thrust and range, reduced fuel consumption, and reduced emissions. Increased energy density typically results in faster ignition and higher burn rates. In addition, nano-particle fuel additives can increase the availability of propulsion grade fuels. For these purposes, the art generally recognizes the term “nano-particle” to refer to particles having three dimensions falling within a nanometer scale, typically, ranging from about 1 nm to less than about 300 nm. Due to their high surface area to volume ratio, nano-particles offer distinct advantages over larger scale particles, such as micro-sized particles; for example, nano-particles offer increased surface area and may improve radiative heat transfer properties.
Nano-particle energetic materials known for their fuel-enhancing properties include nano-particles of various metals and metalloids, their hydroxides, oxyhydroxides, and oxides, among them aluminum oxide and silicon oxide being well recognized. Such particles can be readily mixed with solid fuels and propellants to advantageous benefit. See, for example, the disclosure of H. Tyagi, et al., “Increased Hot-Plate Ignition Probability for Nanoparticle-Laden Diesel Fuel,” Nano Letters, Vol, 8., No. 5., 2008, pp. 1410-1416.
Addition of nano-particle fuel additives to liquid hydrocarbon fuels, such as diesel and JP-8, is less studied, because such particles cannot be solubilized or stably dispersed in the liquid fuel. Even as colloidal suspensions and gels, nano-particles lose stability, agglomerate, and sediment from liquid hydrocarbon fuels. Moreover, metal and metalloid nano-particles can quickly oxidize during combustion and become inactive, resulting in emissions of environmentally unfriendly combustion by-products containing metals or metalloids and with reduced effect on the combustion properties of the fuel. Moreover upon combustion, metal and metalloid nano-particles will leave a detectable signature, which may be undesirable in military operations.
It is known to disperse functionalized graphene as a colloid in nitromethane, a liquid monopropellant, as reported by J. L. Sabourin, et al., in “Functionalized Graphene Sheet Colloids for Enhanced Fuel/Propellant Combustion,” ACS Nano, Vol. 3, No. 12, 2009, pp. 3945-3954. Graphene comprises a 2-dimensional, crystalline allotrope of carbon, in which carbon atoms are densely packed in a regular array of sp2-bonded, atomic scale hexagonal pattern. Graphene can be described as a one-atom thick layer of graphite, as disclosed by H. Schniepp et. al., in “Functionalized Single Graphene Sheets Derived from Splitting Graphite Oxide,” The Journal of Physical Chemistry B, vol. 110, 17, 2006, 8535-8539. In the context of the Sabourin publication, “functionalized graphene” refers to graphene functionalized on its surface with epoxy and hydroxyl groups and on its sides with hydroxyl and carboxyl groups. Hereinafter, these oxygen-containing groups will be referenced collectively as “oxygen functionalization.” Significantly, oxygen functionalization renders graphene polar and therefore compatible with nitromethane, which is a polar liquid at ambient temperature and pressure. In addition, oxygen-functionalized graphene dispersed within nitromethane enhances ignition and combustion rates, as taught by J. L. Sabourin, et al.
The art discloses that oxygen-functionalized graphene cannot be solubilized in non-polar liquid hydrocarbon solvents. In contrast, prior work in lubricant development has found that graphene alkylated with long-chain alkyl groups can be solubilized and dispersed in certain pure organic solvents, such as toluene, hexane, and hexadecane for lubrication applications, as reported by S. Choudhary, et al. in “Dispersion of alkylated graphene in organic solvents and its potential for lubrication applications,” Journal of Material Chemistry, Vol. 22 (2012), pp. 21032-21039.
As mentioned above, liquid hydrocarbon fuels provide an extra challenge in that conventional fuel additives cannot be solubilized or stably dispersed therein. It would be desirable to discover a fuel additive that can be solubilized and/or stably dispersed within a liquid hydrocarbon fuel for enhancing properties of the fuel, preferably, for the purpose of enhancing the fuel's energy density, thrust, flame speed, or combination thereof. It would be desirable for a mixture comprising the liquid hydrocarbon fuel and the fuel additive to be stably maintained in storage for long periods of time, for example, on the order of about one month or longer, without sedimentation or agglomeration of the fuel additive. It would be more desirable if the fuel additive did not contain a metal or metalloid, so as to eliminate environmentally unfriendly combustion by-products and emissions upon combustion. It would be even more desirable if such a fuel additive were itself combusted during the fuel combustion process without leaving a detectable signature.